1. Field of the Invention
The present invention relates to an apparatus for generating an image from a digital video input signal. The apparatus is improved so as to reproduce an image with high quality.
2. Description of the Prior Art
In the past, methods generally referred to as the dither method and the density pattern method have been proposed for reproducing images of half tones. These known methods, however, cannot provide satisfactory gradation of dot size when the size of the threshold dot matrix is small and, therefore, require the use of a threshold matrix having a larger size. This is turn reduces the resolution and undesirably allows the texture of the image to appear too distinctive due to the periodic structure of the matrix. Therefore, deterioration of the quality of the output image results.
In order to mitigate the above described problems, it has been proposed to modify the dither method so as to allow finer control of the dot size by the use of a plurality of dither matrices. This method, however, requires a complicated circuit arrangement for obtaining synchronism of operation between the dither matrices so that the system as a whole is large in size, complicated in construction, and slow. Thus, there is a practical limit in the incremental increase of dot size and the resultant increment of density available by the use of a plurality of dither matrices. In U.S. Pat. No. 3,916,096, a method of improving the conventional screening process is described. As set forth in this U.S. Pat. No. 3,916,096, at column 8, lines 19 through 31:                The conventional screening process when applied to a scanned image can be regarded as a form of pulse-width-modulation whereby a line of length X is laid down and repeated at intervals of Y. The percentage transmission (or reflection) of the reproduced image is then Y−X/Y [sic. should read (Y−X)/Y]. To be a linear process (Y−X) must be directly proportional to the amplitude of the scanned video signal where the signal amplitude represents the percentage optical transmission of the recorded original image. A way of achieving this is by comparing the amplitude of the video signal with a sawtooth wave form and laying a line forming a portion of a dot whenever the sawtooth is larger than the video signal.See also U.S. Pat. No. 4,040,094, which relates to similar subject matter.        
However, even if the method described in this patent is used in an apparatus for reproduction of an image, the precision of gradation reproduction deteriorates due to the delay of response of the apparatus.
The conventional method described in U.S. Pat. No. 3,916,096, produces a linear mapping from the analog video signal to the pulse-width-modulated signal. As is known in the art of printing, this linear mapping does not produce acceptable results because of the non-linear distortions introduced in the half-tone printing process, in particular when used with a laser beam print engine. Therefore, to obtain high quality half-tone printing, a method of non-linear mapping must be found. And, the method disclosed in the noted U.S. Patent, as quoted above, uses a complex arrangement to allow the use of different sawtooth waveforms on successive scans.